It is desired that optical media players operate with plural types of optical media. As a corollary, it is desired that construction of optical media enable supporting diverse optical recording layers in a manner for enhancing optical writing and reading operations while increasing total storage capacity.
It is also desired in magnetooptical disk data storage systems to provide the so-called direct overwrite. That is, rather than the current practice of requiring an erasure of a magnetooptical data storing area before writing or recording in such area, it is desired to provide for direct overwriting any currently stored data in a magnetooptical data storage system, whether single-sided or double-sided media are used.
Several direct overwrite methods are known. On such method for direct overwriting included modulating the bias magnetic field while keeping the laser writing beam at a constant write intensity rather than the current practice of modulating the intensity of the laser writing beam while holding the magnetic field direction constant. A second known method of direct overwrite in MO media is to modulate the laser beam for writing combined with a pair of magnetic bias coils, one coil for writing and one for erasing MO media. Other methods of direct overwrite, some of such methods include particular media constructions termed a "media solution", have been proposed. Such additional methods are not described here.
One of the problems with obtaining such direct overwriting in MO media is that the magnetic field source is disposed a relatively long distance from the magnetooptical recording layer. This spacing not only requires what is considered as excessive magnetic power, but also the cross-section of the magnetic field has a greater dimension than desired for high density recording. To solve these problems, the source of the magnetic bias field must be as close as possible to the magnetooptical recording layer and the laser beam must be accurately focussed at the magnetooptical recording layer.
It is particularly useful to increase the current data storage capacity of smaller diameter optical disks, such as the 90 mm diameter optical disks. Doubling the data-storage capacity of these small diameter small disks provide a cost effective compact data storage unit. For enabling compatibility with current day 90 mm optical disk players, the disk construction for doubling the capacity of each disk should not increase the axial dimension of the disk. Such media construction should also accommodate all types of optical recording layers.